Problem: There are 39 trees on the banks of the river. The right bank has 8 of them. How many trees are on the left bank of the river?
Solution: The number of trees on the left bank of the river is the difference between the number of trees on both banks of the river and the number of trees on the right bank. The difference is $39 - 8$ trees. $39 - 8 = 31$ trees.